The invention relates to an optical switching element comprising two optical guides, whose opposite walls are separated by a small distance d designated as coupling distance and are parallel over a length D designated as coupling length, these optical guides being each constituted by a linear strip G.sub.ii and H.sub.ii, respectively, of a semiconductor material having a first refractive index n.sub.1 formed on a semiconductor substrate of a material having a second smaller refractive index n.sub.o, the dimensions of these guides being such that they each transport a monomode wave and the operation of switching the light from one guide to the other guide being set up by the effect of potentials or currents to which electrode systems E.sub.ii and M applying to the guides are brought.
The invention further relates to an optical switching matrix formed by a number n.times.m of these switching elements.
The invention finally relates to a method of manufacturing the switching element and the matrix.
The invention is used in the switching of optical signals transported by optical fibres, for example in the field of telecommunication, in which the switching between numerous optical fibres must be rendered possible to avoid that the optical signals transported by the fibres must be converted into electrical signals during the operation of switching between the different subscribers.
An optical element apt to form a switching matrix of the kind defined in the opening paragraph is known from the publication entitled: "Double heterostructure GaAs--Al.sub.x Ga.sub.1-x As Rib Wave Guide Directional Coupler Switch" by J. Brandon, A. Carenco et alii in "Second European Conference on Integrated Optics", 17-18, Oct. 1983 in Florence, Italy.
This document describes a switching element constituted by two optical guides each formed by a strip of gallium arsenide (GaAs). These strips are parallel and are formed beside each other at the surface of a layer of gallium aluminium arsenide (GaAlAs), which serves as a layer confining the light in the guides opposite to the flat substrate of gallium arsenide (GaAs). Another layer of gallium aluminium arsenide (GaAlAs) covers each of the strips forming the guides. The latter are formed on the other hand in relief on the substrate by etching a starting layer. Electrodes of the Schottky type are finally formed at the surface of each of the guides, while the opposite surface of the substrate is provided with a layer forming an ohmic contact.
The guides each transport a monomode wave and due to the small distance separating them laterally a polarization applied to one or the other of the guides permits of changing the coupling of the two monomode guides, thus ensuring the desired switching.
However, the coupling length, that is to say the length necessary in order that the light passes from one guide into the other, depends on the one hand upon the coupling distance, that is to say the distance laterally separating the guides, and on the other hand upon the structure of the guides. The structure of the guides known from the aforementioned document is of the "external strip" type. Now the coupling between two such guides is small and therefore the coupling length is great, i.e. typically 5 to 8 mm.
Such distances are much to great to be used for the formation of integrated switching matrices. In fact, a matrix will comprise a number n of such coupling elements arranged in cascade in the longitudinal direction of the guides, which will lead to surfaces redhibitory for the integration.
On the other hand, in order to obtain such a switching matrix, it is necessary to deflect periodically the optical path of one of the waves; this can be obtained by joining one of the guides of one element to one of the guides of another element by means of a curved guide portion, as is shown in the publication entitled: "New Directional Coupler for Integrated Optics" by F. Auracher and H. H. Witte in "Journal of Applied Physics", Vol. 45, No. 11, November 1974, pp. 4997 to 4999. However, in order to present small losses, the curved guide must have a large radius of curvature, i.e. typically ten milimeters, in order to keep the losses below 1 dB, during a change in direction of 90.degree.. The surface area occupied by the matrix is the more substantial and less suitable for integrations.
The latter document proposes to obviate certain disadvantages of such a switching element in that the guides are obtained by means of superimposed layers. However, during use it appears that the masks necessary for the alignment of one guide with respect to the other must be formed with such a high accuracy that the manufacturing efficiency becomes too low for an economical application for integrated circuits. This document thus proposes a structure in which two optical guides are arranged beside each other separated by a distance sufficiently great to avoid a spontaneous coupling and are covered by a layer to obtain a third optical guide straddling the two first guides and separated by a dielectric layer from said guides. This document indicates that in these conditions the coupling is improved. However, in addition to the disadvantage due to the aforementioned alignment of the superimposed guides, the problem is still complicated due to the fact that this structure requires the use of three optical guides. Moreover, no other devices are shown than a discrete element and there is not indicated either how a complete switching matrix should be obtained. Now the designer of circuits knows how difficult it is to pass from the formation of a discrete element to the formation of a complex circuit.
Finally, it is known from European Patent Application EP-A-0 209190 to apply a mirror to one of the guides to deflect the light beam. However, the structure described is constituted by two superimposed guides, which structure has to be avoided for the reasons described above, while on the other hand the formation of a mirror on the upper guide by means of etching comprises very critical masking steps in order to avoid the etching of the lower guide, which renders the formation of this assembly comparatively difficult. Consequently, the efficiency of manufacturing such a device would be low.